The importance of the process of extraction has increased with the growing global awareness of the medical and nutritional benefits of plant and animal isolates. For example, fish oils are extracted to provide healthy fatty acids, fats are separated from milk to reduce its fat content, and flavors, spices and herbal isolates are extracted from plants. In general, these extraction methods are specific for one or two solutes present in the material. Most continuous extraction methods are not chemical in nature, but are mechanical, such as the extraction of seed oils. As such, extraction methods are not complex, and do not provide for the separation of the final solutes.
In the example of cannabis species it has been discovered that compounds within it provide medicinal benefit for such conditions as inflammation, pain, nausea and epilepsy. Until recently, the belief in the “Entourage Effect” meant that the use of a totality of the plant in some way, such as smoking or consuming whole extracts, was necessary to achieve the effects. This has been disproven—for example, the drugs Marinol and Syndros include the active ingredient dronabinol, a synthetic delta-9-tetrahydrocannabinol (THC). Cesamet includes the active ingredient nabilone, which is synthetically derived and has a chemical structure similar to THC.
There are some 60 possible bio-active compounds within the cannabis plant. Some of these compounds are predicted to be valuable natural-source drugs which may not be associated with the plethora of side effects which plague commercial synthetic drugs.
Recently there has evolved methods and solvents used to extract products from cannabis species. Originally, leaves, stems and buds from the plants were dried and smoked. Awareness that solvent extraction could be used to obtain an edible oil product which had similar effects, without the detrimental effects of smoking, led to experimentation with solvents. As more exploration of cannabis took place, it was recognized that there are a plethora of active ingredients in the cannabis buds. It has been long thought that to get the benefit of cannabis, the consumption or smoking of all of its compounds was necessary, resulting in what has been termed the Entourage Effect.
Early extraction used naphtha, ethyl ether, butane, 99% isopropyl alcohol, or hexane. When cannabis use increased, commercial equipment was marketed. Subsequent extractors used propane or butane, or a mixture of both, to extract the cannabis oils. Post-extraction treatment of the oils, such as cooling or heating, or adding ethanol to the oil, produced quirky physical forms of the extracts, such as shatter, wax, errl, oil, live resin, or dabs. However, there is always the propensity for some residue of the solvent used in the extraction process remaining in the end product. In the case of ethanol, this may not be toxic, but it is nevertheless undesirable.
As legalization occurred, cannabis use also has increased, and more effort has been made to regulate the quality of the cannabis products. For example, solvent, fertilizer and herbicide residues have been restricted. The difficulty of obtaining a residue-free product was the impetus for further development of the extraction process. The use of supercritical fluids was found to be an alternative technique that produces extracts with none or fewer polar impurities than the conventional organic liquid extracts. It is considered to be a green technology, since a concentration step is most often eliminated after the extraction process.
For any solvent, its vapor/liquid equilibrium curve culminates at the critical point (CP), above which only one phase occurs. All solvents possess a CP, which is characterized by a critical temperature (Tc) and a critical pressure (Pc). Experimental studies using SFE are usually limited to the region of Pc<P≤6Pc and Tc<T≤1.4Tc.
The most commonly used supercritical fluid is CO2 because it has favorable Tc and Pc (31.1° C. and 73.9 bar) that are ideal for the extraction of thermolabile compounds. In addition, supercritical CO2 has low viscosity, low surface tension, high diffusivity and good density and is also non-toxic, non-flammable, cheap, widely available, chemically inert under several conditions, and gaseous at normal pressure and temperature, eliminating the step of solvent evaporation after extraction. Furthermore, CO2 gives a non-oxidizing atmosphere in extractions, thus preventing extracts from degradation.
The greatest limitation of supercritical CO2 is that it is not suitable to extract polar compounds. It is necessary to add an organic modifier or entrainer, such as ethanol or methanol to greatly improve extraction efficiency. The use of methanol may result in unacceptable residues in the product.
Other solvents, such as water, methanol, ethanol, acetone, chloroform, ethyl acetate, and toluene, are not appropriate to extract bioactive compounds, because their Tc is above 200° C.